[ENH] Generalize all custom plugins to accept a schema name

[charlesbluca]

Is your feature request related to a problem? Please describe. Currently, many of the custom plugins (such as CREATE MODEL) are not supported for non-default schemas, operating directly on context.schema_name.

Describe the solution you'd like It would be nice if all custom parsing functions in parser.rs parsed the schema name (if provided) for use in the Python plugins, using either elements_from_tablefactor or some other helper function.

Describe alternatives you've considered An alternative is to use USE SCHEMA statements to change the underlying context.schema_name; however, this doesn't necessarily resolve the potential user confusion around the mixed support of schema names.